The plasma processing system may be used in the following way. A fluorine-based gas and a workpiece, such as semiconductor wafers or liquid crystal substrates, are placed in the chamber of a plasma processing apparatus. In the chamber a pair of electrodes are provided, and radio-frequency power from the radio-frequency power source is provided to the pair of electrodes for generating electric discharge. This discharge generates a plasma of the gas contained, and thin film forming or etching is performed on the workpiece.
Conventionally, as a radio-frequency power source used in a plasma processing system, the following type is known. A radio-frequency power source outputs radio-frequency power of a predetermined frequency, and the output power is pulse-modulated based on pulse modulation control signals having a low frequency than that of the output power. In such radio-frequency power source, the radio-frequency power is outputted only during the high level period of the pulse modulation control signals, and there is no output during the low level period of the signals. Accordingly, the resulting radio-frequency power has a pulsed form (see, for example, Patent Document 1).
In addition to the above-described ON/OFF control in which the output and non-output states of the radio-frequency power are switched, there is also known a control method in which the amplitude of the radio-frequency power is switched between two levels, i.e., a first level and a second level which is lower than the first level. In this case of two-level control, for example, the voltage supplied to the amplifier is subjected to the switching between the two levels, so that the power outputted from the amplifier has corresponding two levels switched to form a pulsed output.